Radio transmitter and receiver circuits usually comprise frequency synthesizers to generate up-conversion frequencies and down-conversion frequencies that are used to up-convert signals to be transmitted and down-convert received signals, respectively. The frequency synthesizers may be implemented comprising a voltage-controlled oscillator (VCO) or a digitally controlled oscillator (DCO). Further, for operating the transmitter and receiver circuits a supply voltage is required. The supply voltage is often provided by a DC/DC converter which, for example, converts a battery voltage to a lower supply voltage. The DC/DC converter may be a switch-mode DC/DC converter which performs the voltage conversion by applying a DC voltage across an inductor or capacitor for a period of time which causes current to flow and store energy magnetically or electrically, then switching this voltage off and causing the stored energy to be transferred to the output of the converter in a controlled manner.